The present invention relates generally to metal composite wire or sheet structures containing low-temperature (low-T.sub.c) or high-temperature (high-T.sub.c) superconducting (HTSc) ceramic oxides. In these structures, the superconductor is deposited as a deposit, within a metal composite matrix with or without additional metallic, metal alloy, or ceramic filamentary components. The present invention relates, in particular, to the design, process fabrication and construction of HTSc-metal composite wires or sheets of finite or indefinite continuous length, in which the HTSc ceramic is embedded within or deposited on a metallic sheet that is used in the final composite structure. The superconducting ceramic is used either as a single filament (superconducting layer), or as multiple filaments, or as either in association with other metallic or ceramic filamentary components that provide greater mechanical, thermal or electrical stability to the function of the composite structure. Furthermore, each filamentary component containing HTSc ceramic is imparted a predetermined c-axis orientation relative to the longitudinal or radial axis of the wire, or planar axis of the sheet. Application of this c-axis textured ceramic sheet allows the designer to improve the electromagnetic performance characteristics of the composite structure.
Superconductors conduct electromagnetic power without resistive loss when cooled below an intrinsic thermodynamic transition temperature, commonly defined as the T.sub.c of the material. In addition to allowing the resistanceless transport of electromagnetic power, the superconductive state also nulls electric fields and expells magnetic flux (lines of force) from within the interior of the material. This combination of properties allows superconductors to be useful in a variety of electromagnetic systems that require electromagnetic power storage, power delivery, power regulation, low loss power transmission, power amplification, or electromagnetic shielding.
Electromagnetic power can be stored within a superconducting wire or sheet when it is wound or formed into a topological surface representative of a magnetic coil or solenoid. Power is supplied to the superconductor by driving an electrical current through it from an external source. This generates an electrical supercurrent within the superconductor. This is known as charging the superconductor. When the superconductor is sufficiently charged, a superconducting short circuit is activated between the ends of the coil structure, so the electrical supercurrents circulating within the coil may follow a closed loop continuous superconductive path. Since the superconducting path does not dissipate the electrical power, the supercurrents persist indefinitely. After the superconducting storage device as been charged, the stored power can be tapped to supply power to an electrical grid.
This same configuration can be used to regulate power to the grid when active electrical switches and relays that monitor the grid are configured to tap the electrical power in the superconductor when the grid is experiencing an electrical energy surplus. Since the superconductor is an extremely low loss electrical power conduit, cables, bus bars, or leads, can also be used to transmit power from an electrical energy source to the load with negligible power loss, or power loss that is significantly less than that of normally resistive power feeds. Since the superconductor has no dc electrical resistivity, it can be used to transfer electrical power in a cryogenic environment with negligible heat generation.
Radio frequency (rf) electrical power is amplified by a cavity resonator. The physical dimensions and dielectric constants of the materials used to construct the cavity determine the frequencies that will maximally experience power amplification within the resonator. Quality-factors (Q-factors) characterize the gain per unit frequency within the resonator structure. Q-factors can be greatly enhanced, allowing significant improvements to rf gain over a narrower band of rf frequencies if the walls of the cavity structure are coated with a layer of superconducting material.
Since the superconducting state thermodynamically prevents electric and magnetic fields from penetrating its interior, a hollow superconducting surface enclosing field sensitive instrumentation can shield these devices from harmful external electromagnetic radiations. Likewise, if a magnetic field source or electromagnetic radiation source is placed within a hollow superconducting shell, the superconducting surface can be used to confine or constrain field emissions or to shape magnetic field emissions protruding out a hole in the superconducting shell.
These embodiments of superconducting topologies are useful in a variety of electrical systems that require the efficient utilization of the available power budget, such as an airborne or space-based system. They are also useful in devices that require the efficient manipulation, generation, or delivery of powerful electromagnetic pulses, for instances in electromagnetic weaponry or electromagnetic rail guns; or, the regulation, storage and transmission of electric power over an electrical grid; or, the amplification of an rf power source, for instance in electric countermeasure devices or radar systems; or to protect sensitive electronic equipment against electronic countermeasures or interfering radiation.
All of these applications require the superconducting material to be cooled to a temperature below its thermodynamic transition temperature. If the material is heated above its T.sub.c it will fail to operate as a superconductor and revert back to its state of normal resistance, causing the unique functionality of the material to be lost to the application. The application of magnetic fields and electrical currents to the superconductor can also stress the thermodynamic state of superconductivity. If the superconductor is maintained at the lowest possible temperature and no electrical currents are passing through it, a threshold magnetic field can be applied above which magnetic flux is no longer expelled from the material and it fails to remain superconducting. The value of magnetic field that causes the superconducting state to rupture is defined as the critical magnetic field (H.sub.c). If the temperature of the superconductor is increased to a value that is elevated but still below its T.sub.c the intensity of an applied magnetic field that will rupture the superconducting state is less than the H.sub.c measured at the lowest possible temperature. The intensity of magnetic field that is needed to rupture the superconductor is an increasing function of decreasing temperature below T.sub.c of the superconductor. This relationship can also be interpreted as meaning the T.sub.c of the superconductor is a decreasing function of increasing applied magnetic field intensity.
The physical representation of this relationship can be mapped onto a graph using temperature and magnetic field intensity as axes, with a line defining the boundary between superconducting and normally resistive states of the material. This line is referred to as the irreversibility line of the superconductor. All values of field and temperature that are within this line (closer to the origin of the axes) allow the material to retain its superconductive properties. All values of field and temperature that are exterior to this line (further from the origin of the axes) rupture the superconductive state of the material.
A similar functionality is observed with electrical current traveling through the superconductor. At the lowest possible temperature, and in the absence of an applied or generated magnetic field intensity, the superconductor will be able to transport a maximal level of electrical current density. If the current density is increased beyond this maximal level, known as the critical current density (J.sub.c) of the superconductor, the superconductive state is ruptured. As is the case with applied magnetic field intensity, the J.sub.c decreases with increasing temperature below T.sub.c. It can alternatively be stated that the T.sub.c of the superconductor decreases with increasing current load.
Superconductivity was initially discovered in certain pure metals (such as mercury, lead, vanadium, niobium and tin) that are cooled to very low temperatures, generally less than 4 K. These superconductors, known as type-I superconductors, expell all magnetic flux from their bulk interiors until a critical magnetic field intensity is applied. The critical magnetic field intensities, critical current densities, and critical temperatures of these superconducting pure metals are so low that utilizing the phenomenon of superconductivity with them has limited practical value.
It was subsequently discovered that certain impure metals and metallic alloys, like niobium-tin or niobium-germanium, also exhibit superconductivity at elevated temperatures and higher magnetic field intensities. What distinguishes these superconductors from the type-I superconductors is that the superconducting state still persists even though some of the applied magnetic flux actually penetrates their interior without rupturing the superconducting state. The penetrating flux is confined to tubular clusters known as fluxoids. In normally resistive material all of the lines of applied magnetic flux are uniformly distributed throughout the material. In this mixed state of a superconductor the penetrating flux lines are packed into discrete tubular flux clusters. Equal groupings of flux lines are channeled into the fluxoids, causing the flux line density to increase within the fluxoid and be zero between fluxoids. Inside the volume of the fluxoid the superconductor is in a state of normal electrical resistance, while outside the microscopic volume of the fluxoid it retains its superconducting properties.
Under stable conditions of thermodynamic equilibrium, these bundles of magnetic flux distribute themselves with uniform cluster densities over the superconducting surface and maintain their equilibrium positions. A larger fraction of the superconductor's volume remains superconducting if fewer fluxoids penetrate the superconductor. Fluxoid penetration increases with increasing superconductor temperature below T.sub.c. Superconductors that exhibit this "mixed state" are known as type-II superconductors. These materials have greater practical use since they retain their superconducting properties at higher temperatures than type-I superconductors, and at magnetic field intensities common to many practical application.
Superconductivity has also been observed in certain ceramic materials, such as barium-potassium bismuthate (Ba.sub.x K.sub.1-x BiO.sub.3) and a variety of copper-oxide ceramics, including yttrium-barium-copper oxide (YBa.sub.2 CU.sub.3 0), and specific material phases of certain bismuth cuprate (Bi-Pb-Sr-Ca-Cu-O), thallium cuprate (T1-Ba-Ca-Cu-O), and mercury cuprate (Hg-Ba-Ca-Cu-O), ceramics. The cuprate superconductors exhibit type-II superconductivity at significantly higher superconducting transition temperatures (90-140 K), and are referred to as high-T.sub.c superconductors. The bismuthate ceramic is an isotropic low-T.sub.c superconductor.
It is thus defined that the superconducting state is a function of the material, its temperature, the current density flowing through it, and the magnetic field intensity applied to it. In order for a superconductor to be applied in any of the applications mentioned above, the environment in which it is operating must be designed to maintain thermodynamic conditions that prevent the material from transitioning from the superconducting state into its state of normal electrical resistance. The ability to maintain control over the thermodynamic state of the superconductor can be improved by enveloping the superconductor as a single, or as multiple, filamentary strands(s) within a metal composite matrix. The specific metals or metallic alloys, as well as their relative physical dimension, used in this composite structure are selected on the basis of their intrinsic physical properties and the ability of these properties to relieve or mitigate the occurrence and propagation of energetic disturbances or instabilities that develop within the superconductor as it is operated.
Energetic disturbances that can compromise the performance of active superconductors are known to have either a mechanical origin or be the result of magnetic flux jumping. Mechanical disturbances could be due to the gradual or catastrophic release of physical stresses that develop as a result of electromagnetic loading on the mechanical component(s) of the superconductor, or due to transient or steady state vibrations that develop within the superconductor as electrical current passes through it. Magnetic flux jumping describes the sudden and dissipative rearrangement of magnetic flux within a superconductor. It is predominantly generated by the repulsive electromagnetic interaction of the penetrating lines of magnetic flux with electrical currents transported through the superconductor. Magnetic flux oriented perpendicular to the path of moving electrical charge is subject to an electromagnetic force, known as the Lorentz force, F.sub.L.
In isotropic superconductors, the magnitude of the Lorentz force on a single bundle of penetrating magnetic flux is: EQU F.sub.L =.PHI..sub.0 sin .crclbar.. (1)
where .theta. is the current flow per unit area passing by the flux bundle, .PHI..sub.0 is the magnetic flux density contained within the bundle, and .crclbar. is the angle subtended between the direction of the current flow and the orientation of the magnetic flux bundle. The effect of this force on the electrical current causes it to be deflected from its original path. The effect of this force on stable penetrating flux bundles causes them to be dislodged from their equilibrium positions. The acquired kinetic energy of the moving fluxoids is eventually dissipated within the superconductor as heat.
Often these energetic disturbances occur at specific points within the superconducting structure. If the energetic point disturbance is sufficiently long lived, the accumulated thermal energy dissipated by the disturbance may be sufficient to locally trigger the superconducting state to revert back to its state of normal resistance at that point. Current passing through this normally resistive point will start generating greater quantities of heat, which, if not quickly transported to a thermal reservoir, can precipitate catastrophic failure along the entire length of the conductor. An objective for embedding the superconductor within a metal composite structure is to provide thermally conductive pathways that drain the dissipated heat at rates faster than the energetic disturbance can generate it in the superconductor. Superconducting metal composite structures are also useful because the metal provides a low resistivity electrical pathway to shunt current in the superconductor if it goes normal. Filamentary components can also be selected to improve the mechanical integrity of the composite through strength membering, or to dampen unwanted vibrational modes, thus relieving the incidence of mechanical energetic disturbances.
The selection of metallic components interfaced with the superconductor must also be compatible with its material processing requirements. The sintering kinetics of high-T.sub.c superconducting ceramics require that the ceramic precursor be hermetically encased in a silver sheath if they are to be efficiently processed into the more desirable superconducting phases. Silver has very high rates of oxygen diffusivity at elevated temperatures. Exposing the ceramic to controlled oxygen atmospheres is needed to regulate the thermal reaction kinetics while processing the precursors into the desired phase of the sintered ceramic. Some components of high-T.sub.c superconducting ceramics can become volatile in oxygen partial pressure atmospheres that are favorable to the reaction when elevated to the necessary process temperatures. The hermetic silver barrier blocks evaporative or liquid loss of the more unstable precursor components while still permitting use of optimal oxygen atmospheres to favorably regulate the chemical kinetics of the sintering reaction.
Composite wire structures of high-T.sub.c superconducting ceramics are currently manufactured by billet processing techniques. In billet processing, a stoichiometric blend of precursor powder, referred to as the billet, is packed into a silver tube which is hermetically sealed at both ends. The billet can either be a blend of elemental metallic precursors, elemental metal oxide precursors, or powders of distinct microstructural phases of the ceramic that are known to react favorably or efficiently into the desired single phase of the ceramic during subsequent processing. Depending upon which billet process is used, this tubular composite structure can then be mechanically deformed and elongated into a monocore wire and reacted to form the superconducting phase, or pre-reacted and drawn into monocore wire structures before the final thermal sintering of the superconductor.
Existing billet-processing techniques allow precursor powders to be deformed into monocore (single filament) wires. Multifilamentary wire structures produced using this technique are fabricated by fusing multiple monocore powder-in-tube structures into a large bundle, and drawing the entire composite through a die to reduce its overall diameter and to extend its length before reacting the formed wire. In both cases, the superconductor is in intimate contact and fully enveloped by the silver casing.
Often it is desirable to have metals other than silver in intimate contact with the superconductor to reinforce its thermal or mechanical performance. The selection of what metallic composition would be usefully applied in intimate contact with the superconductor depends upon whether or not it would be more useful to improve the thermal or mechanical performance of the superconductor in the application for which the composite structure is intended. Since precursors powders are necessarily packed into a sealed tube, billet processing techniques only allow silver, or metals with comparable oxygen diffusivities at temperatures used to process the superconductor, to have intimate contact with the superconductor. The composite structures made using these metals and this technique may not allow the superconductor to achieve optimal performance standards within the composite.
Billet processing techniques have other practical drawbacks. While they can be used to synthesize long lengths of wire, it is not a processing technology that is well suited for many of the application listed above. For instance, high-T.sub.c superconducting electrical power transmission between a power station and a substation will require extremely long continuous lengths of superconductor. Billet processing only permits the fabrication of composite structures of finite length determined by the initial dimensions of the tube into which the billet is packed and the extent to which it is deformed. Other possible applications, such as electromagnetic shielding or superconducting rf cavity resonators may require large continuous sheets of high-T.sub.c superconducting ceramic, or exposed ceramic surfaces that cannot be made using billet processing.
Another approach to improving the performance of the superconducting composite structure is to implement design structures that reduce the incidence of magnetic flux jumping in the superconductor. Flux jumping can be hindered by introducing microscopic defects that act as flux pinning centers. Fluxoid bundles are immobilized at these centers, and consequently will not dissipate kinetic energy into the superconductor provided the flux pinning potential of the center is greater than the energies the fluxoids are experiencing through Lorentz force interactions with currents in the superconductor. In multiple element superconductors, I.e., not pure metals, flux jumping can be reduced by fabricating the superconductor using a materials synthesis process that achieves fine subdivision of the individual precursors. Powder synthesis techniques do not allow extremely fine precursor subdivision.
A more fundamental problem to billet processing is its inability to implement innovative composite design architectures that reduce magnetic flux jumping in the high-T.sub.c superconducting ceramics. When the billet is packed into the silver tube, the crystallographic orientation of the precursor powders are randomly oriented with respect to one another. The final ceramic reaction product is c-axis textured by thermomechanically sinter processing--(heating and flattening)--the entire composite structure. Consequently, as a result of this thermomechanical sintering step, the finished phase in the ceramic filament(s) is produced with its c-axis oriented uniquely in one direction.
A fundamental property of the high-T.sub.c superconducting ceramics is their crystallographic anisotropy. The crystallographic structure of these ceramics is characterized by the crystallographic c-axis, the long axis of its crystalline unit cell, which is perpendicular to the basal or a-b-crystallographic plane. Superconductivity in the high-T.sub.c cuprates is exhibited primarily along the basal plane of the ceramic. Superconductive phenomena, as measured by critical current densities and critical field intensities, is considerably weaker in these ceramics along orientations parallel to the crystallographic c-axis. This is particularly true at elevated temperature and in applied magnetic fields. Consequently, these ceramics are more susceptible to flux jumping in magnetic fields that are oriented parallel to the ceramic c-axis than they are to magnetic fields oriented along their basal plane.
This anisotropy is less exagerated at low temperatures, i.e., near 4 K, but becomes increasingly and dramatically pronounced at temperatures above 10-15 K. Therefore, despite the high superconducting (onset) transition temperatures for these ceramics (90-127 K), this fundamental property can severely limit their practical application unless superconducting surfaces and multifilamentary composites can be constructed with predetermined c-axis topologies that favorably orient the ceramic to the anticipated magnetic field lines of a given application. Depending upon the application, this will require some ceramic filaments in the structure to have their c-axis oriented in directions that are different from others. This cannot be achieved using billet processed composites in which c-axis texturing is achieved, if it all, during the final sintering step, usually causing all of the ceramic filaments to have similar c-axis orientation if they are thermomechanically processed.
Thus, there exist a need for a more effective manufacturing process to construct high-T.sub.c superconducting multifilamentary composite structures. The improved manufacturing process should allow superconducting ceramic components to have predetermined c-axis orientation within the composite structure, as well as provide a means to place the superconducting components in intimate contact with filamentary metals or metal alloys other than silver without sacrificing an oxygen diffusion pathway to the exterior of the composite structure during atmosphere-controlled thermal processing. This manufacturing process should also allow other ceramic filamentary components, which have electrically resistive or magnetorestrictive properties that are favorable to the stable function of the composite, to be incorporated with similar ease. Finally, this manufacturing process should also allow composite structures to be fabricated to indefinite continuous lengths or surface areas.